The invention relates to a moisture-absorption sheet, a method of manufacturing a moisture-absorption sheet and an organic electroluminescent device comprising such a moisture-absorption sheet.
U.S. Pat. No. 5,124,204 discloses a moisture-absorption sheet comprising an organic polymer and comprising particles of moisture-absorbing material dispersed into the organic polymer. The moisture-absorption sheet as disclosed herein comprises an organic polymer sheet (examples are given for natural polymers, modified natural polymers and synthetic polymers) with silica gel powder dispersed therein at a certain surface density. The silica gel powder is a drying substance, which binds water.
U.S. Pat. No. 5,124,204 further describes the use of such a moisture-absorption sheet in a thin-film electroluminescent (EL) panel. The EL panel comprises a light permeable base plate, a thin-film EL element formed on the base plate and a moisture-proof sheet covering the thin-film EL element in order to prevent moisture reaching the EL element. Said EL element is further protected from moisture by said moisture-absorption sheet which is placed between said thin-film EL element and said moisture-proof sheet. Alternatively, a moisture-absorption sheet can also be obtained by providing the moisture-proof sheet with a moisture-absorption layer on the surface facing the thin-film EL element.
This prior-art moisture-absorption sheet has the drawback that, in practice, an increase of the quantity of moisture-absorbing material leads, to a reduction of the integrity of the moisture-absorption sheet and causes the moisture-absorption sheet to be susceptible to crack formation and disintegration.
It is, inter alia, an object of the invention to provide a moisture-absorption sheet, comprising an organic polymer that entangles and holds the particles of moisture-absorbing material, in spite of a high content of this material.
To this end, a first aspect of the invention provides a moisture-absorption sheet as defined in claim 1. A second aspect of the invention provides several methods of manufacturing a moisture-absorption sheet, as defined in claims 8, 9 and 10. A third aspect of the invention provides an organic electroluminescent device as defined in claim 11, comprising a moisture-absorption sheet according to the invention. Advantageous embodiments are defined in the dependent claims.
The moisture-absorption sheet of the invention comprises particles of a moisture-absorbing material which are bonded by a relatively small fraction of a polymer having a high weight-averaged molecular weight. The polymer is an organic Ultra-High-Molecular-Weight (UHMW) polymer having a weight-averaged molecular weight of more than 0.1 million. The UHMW polymer interconnects the particles of the moisture-absorbing material in such a way that the moisture-absorption sheet, in spite of the high content of moisture-absorbing material, is less susceptible to crack formation and disintegration. The result is thus, that the moisture absorption sheet of the invention is self supporting. Such self-supporting sheets are highly preferred because they are easy to handle. For example, a predetermined shape can be cut out of the sheet, which shape determines the dosage and the distribution of the moisture-absorbing material.
Using a UHMW polymer according to the invention, moisture-absorption sheets having a large fraction of particles of moisture-absorbing material could be manufactured. In a preferred embodiment of the invention, the moisture-absorption sheet comprises 80 to 99.9 wt. % (weight percentage) particles of moisture-absorbing material, and 0.1 to 20.0 wt. % UHMW polymer. More preferably, the moisture-absorption sheet comprises 95 to 99.9 wt. % particles of moisture-absorbing material, and 0.1 to 5.0 wt. % UHMW polymer.
A further advantage of the moisture-absorption sheet according to the invention is that the moisture-absorption sheet may have a high porosity. A porosity of at least 30 volume % can easily be obtained in a moisture-absorption sheet according to the invention. This porosity makes much more particles of a moisture-absorbing material easily accessible for water molecules than a consolidated moisture-absorption sheet. Due to the porosity, the moisture-absorbing substance has improved moisture-absorbing properties.
In a preferred embodiment, the polymer comprises a UHMW polymer having an average molecular weight of more than 1 million. A small quantity of such a UHMW polymer is able to interconnect the particles of a moisture-absorbing material forming a thin flexible self-supporting moisture-absorption sheet which, in spite of the high content of moisture-absorbing material, is less susceptible to crack formation and disintegration.
The UHMW polymer is preferably a linear polymer. Such a linear polymer with a high weight-averaged molecular weight forms entanglements at relatively low concentrations of the polymer in a solution, which leads to the formation of a network holding the particles of moisture-absorbing material together.
To obtain a moisture-absorption sheet having a high temperature stability, it is desirable for the polymer to be cross-linked. Cross-linking is effected by irradiation with energy in the form of, for example, electromagnetic radiation, ionizing radiation or an electron beam. Said conversion is preferably effected by means of UV radiation.
The organic polymer having a high weight-averaged molecular weight is preferably selected from the group of polyethene, polypropene, polystyrene, polyethene oxide, polypropene oxide, polyacrylate, polycarbonate, polymethylmethacrylate and copolymers thereof. The organic polymer is preferably a polyolefin compound having a high weight-averaged molecular weight.
For the polyolefin compound having a high molecular weight, use is preferably made of Ultra High Molecular Weight Polyethene (UHMWPE) or Ultra High Molecular Weight Polypropene (UHMWPP) having a weight-averaged molecular weight of 1 million or more.
This polyolefin compound may additionally be composed of a mixture of a first polyolefin, such as polyethene, having a high molecular weight, and a second polyolefin having a low molecular weight. The second polyolefin may be the same as or different from the first polyolefin.
It is alternatively possible, however, to use a mixture of a first polyolefin with a high molecular weight and a second polyolefin with a high molecular weight, which second polyolefin is different from the first polyolefin, for example, a mixture of UHMWPE and UHMWPP. Such a mixture exhibits a high resistance to high temperatures and has a low shrinkage.
Various moisture-absorbing materials may be used for the moisture-absorption sheet according to the invention such as, for example, silica gel powder or molecular sieve powder. The moisture-absorbing material preferably comprises a solid compound, which chemically absorbs the moisture and maintains its solid state even after absorbing the moisture. For example, an alkaline metal oxide compound, an alkaline earth metal oxide compound, a sulfate compound, a metal halide or halogenide compound, a perchlorate compound, an organic compound, and zeolites are available as such solid compounds of the moisture-absorbing material. Moisture-absorbing materials such as Calcium oxide, Barium oxide, Magnesium oxide, Potassium, Zelolite Sodium Alumino-Silicate, Silica gel or P2O5are preferred.
The invention also relates to a method of manufacturing a moisture-absorption sheet. It was found that when using the organic polymer having a high weight-averaged molecular weight to bind the particles of moisture-absorbing material, uncomplicated manufacturing methods as defined in claims 8, 9 and 10 can be used.
The invention also relates to an organic electroluminescent device which is contained in a sealed container, which container is provided on the inside with a moisture-absorption sheet of the invention. U.S. Pat. No. 5,124,204 describes (in conjunction with FIG. 1) a conventional organic electroluminescent device which is prepared by forming, in this order, a lower transparent electrode (3), a lower insulation layer (4), an electroluminescent layer (5), an upper insulation layer (6) and an upper electrode (7) on a glass base plate (2). The constitution of the layers (3 to 7) is called an EL element (1), where it should be noted that in some EL elements, the electroluminescent layer (5) may function as insulation layer instead of the lower (4) and/or upper (6) insulation layer. In order to prevent moisture from reaching the EL element, it is covered by a sealing sheet (12) which is adhered to the glass base plate (2) by an adhesive (9), such as an epoxy resin. The sealing sheet (12) is composed of a moisture proof sheet (13) and a moisture-absorption sheet (14).
In order to obtain a highly reliable organic electroluminescent device, the moisture-absorption sheet should have a large quantity of moisture-absorbing material in order to be able to absorb moisture during the whole lifetime of the organic electroluminescent device. As already mentioned, the prior-art moisture-absorption sheet has the drawback that, in practice, an increase of the quantity of moisture-absorbing material leads to a reduction of the integrity of the moisture-absorption sheet and causes the moisture-absorption sheet to be susceptible to crack formation and disintegration. Due to this disintegration, particles of moisture-absorbing material may come in contact with the EL element, which may lead to degradation of the EL element. This brings us to a further advantage of the moisture-absorption sheet according to the invention: the moisture-absorption sheet according to the invention reliably binds and secures all particles of moisture-absorbing material. This advantage is particularly useful for the use of the moisture-absorption sheet in an organic electroluminescent device, as defined in claim 11.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.